1. slides created by Alyssa Harding
CSE 143 Lecture 21
Iterators
slides created by Alyssa Harding

2. AbstractIntList review
Recall our AbstractIntList class
public abstract class AbstractIntList implements IntList {
public abstract int indexOf(int value);
public boolean contains(int value)
return indexOf(value) >= 0; }
...
It contains both methods with bodies and abstract methods

3. AbstractIntList review
Now let’s examine the complexity of some of the methods
Recall that time complexity refers to the growth rate of how long code takes as the input size increases
O(1): no matter how much input there is, we only have to do one step
O(n): if there is n amount of input, we’ll have to do n steps

4. isEmpty() complexity
What’s the complexity of isEmpty() for ArrayIntList?
public boolean isEmpty()
return size() == 0; }
Its size() method returns the size field
This is O(1)

5. isEmpty() complexity
What’s the complexity of isEmpty() for LinkedIntList?
public boolean isEmpty()
return size() == 0; }
Its size() method loops through all the nodes to calculate the size
This is O(n)

6. contains() complexity
What’s the complexity of contains() for ArrayIntList?
public abstract int indexOf(int value);
public boolean contains(int value)
return indexOf(value) >= 0; }
contains() calls indexOf(), which looks at every value
This is O(n)

7. contains() complexity
What’s the complexity of contains() for LinkedIntList?
public abstract int indexOf(int value);
public boolean contains(int value)
return indexOf(value) >= 0; }
contains() calls indexOf(), which looks at every value
This is O(n)

8. equals() complexity
What’s the complexity of equals() for ArrayIntList?
public boolean equals(Object other) {
if ( other instanceof IntList ) { IntList otherList = (IntList)other; if ( this.size() == otherList.size() ) { for (int i = 0; i < this.size(); i++) { if ( this.get(i) != otherList.get(i) ) return false; } return true; } } return false; }
The most repeated line of code is getting a value from the list, so this is our most significant line

9. equals() complexity
What’s the complexity of equals() for ArrayIntList?
If there are n values, we’ll loop through n times
This is O(n)
For each value, we get the value from the array
This is O(1)
Overall, this is O(n) * O(1) = O(n)

10. equals() complexity
What’s the complexity of equals() for LinkedIntList?
If there are n values, we’ll loop through n times
This is O(n)
For each value, we get the value by looping through the nodes in the list
Overall, this is O(n) * O(n) = O(n2)

11. Complexity comparison
Ouch!
Our code worked pretty well for ArrayIntList, but not LinkedIntList
Can we both reduce redundancy and maintain efficiency?
method
ArrayIntList
LinkedIntList
isEmpty()
O(1)
O(n)
contains()
equals()
O(n2)

12. Iterator<E>
Java gives us an Iterator<E> that we can use to loop through the values in a collection
public interface Iterator<E> {
// returns true if another value exists
public boolean hasNext();
// returns the next value in the collection
public E next();
// removes the current value
public void remove(); }

13. Iterable<E> interface
Java also gives us a way of letting it know that a collection can be iterated over
public interface Iterable<E> {
public Iterator<E> iterator(); }

14. Iterable<E> interface
Any class that implements this interface must have an iterator
public interface IntList extends
Iterable<Integer> {
... }

15. Iterable<E> interface
First we’d need to write iterators for ArrayIntList and LinkedIntList
What would we need to keep track of for the ArrayIntList iterator?
the index we’re examining
How fast would it be to look at the next value?
We just increment the index
O(1)

16. Iterable<E> interface
First we’d need to write iterators for ArrayIntList and LinkedIntList
What would we need to keep track of for the LinkedIntList iterator?
the node that we’re examining
How fast would it be to look at the next value?
We just increment the index
O(1)

17. Iterator example Now we can use an iterator to loop through our list:
IntList list1 = new ArrayIntList(); list1.add(7); list1.add(42);
Iterator<Integer> it = list.iterator();
while (it.hasNext()) {
System.out.println(it.next()); }

18. Iterator example
Now we can also use a foreach loop with any class that is Iterable:
foreach (int i : list)
System.out.println(i);
This accomplishes the same thing as the previous slide, but looks much cleaner

19. indexOf()
Now we can use iterators for indexOf() for any AbstractIntList
public int indexOf(int value) {
Iterator<Integer> it = iterator();
int i = 0;
while ( it.hasNext() ) {
if ( it.next() == value )
return i; }
return -1;

20. isEmpty()
Instead of calculating the entire size, we can check whether our list has any elements
public boolean isEmpty()
return !iterator().hasNext(); }

21. equals() The next method is more complex:
public boolean equals(Object other) {
if ( other instanceof IntList ) {
Iterator<Integer> it1 = this.iterator();
Iterator<Integer> it2 = (IntList)other.iterator(); while ( it1.hasNext() && it2.hasNext() ) {
if ( it1.next() != it2.next() ) return false; } return !it1.hasNext() && !it2.hasNext(); } } return false; }

22. Complexity revisited Now they’re the same! method ArrayIntList
LinkedIntList
isEmpty()
O(1)
contains()
O(n)
equals()